Display device

ABSTRACT

An object is to provide a display device that performs accurate display. A circuit is formed using a transistor that includes an oxide semiconductor and has a low off-state current. A precharge circuit or an inspection circuit is formed in addition to a pixel circuit. The off-state current is low because the oxide semiconductor is used. Thus, it is not likely that a signal or voltage is leaked in the precharge circuit or the inspection circuit to cause defective display. As a result, a display device that performs accurate display can be provided.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device, a displaydevice, a liquid crystal display device, and a light-emitting device anda method for manufacturing these devices. In particular, the presentinvention relates to a semiconductor device, a display device, a liquidcrystal display device, and a light-emitting device each of whichincludes a circuit having a transistor including an oxide semiconductorfilm, and a method for manufacturing these devices.

2. Description of the Related Art

Thin film transistors (TFTs) in which silicon layers formed usingamorphous silicon or the like are used for channel layers have beenwidely used as switching elements in display devices typified by liquidcrystal display devices. Although thin film transistors formed usingamorphous silicon have low field-effect mobility, they have an advantagethat larger glass substrates can be used.

Moreover, attention has been recently drawn to a technique by which atransistor is manufactured using a metal oxide with semiconductorproperties and such a transistor is applied to an electronic device oran optical device. For example, it is known that some metal oxides suchas tungsten oxide, tin oxide, indium oxide, and zinc oxide havesemiconductor properties. Patent Document 1 discloses a transistor inwhich a transparent semiconductor layer formed using such a metal oxideis used as a channel formation region.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2006-165532

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide asemiconductor device or the like with less noise, to provide asemiconductor device or the like with high withstand voltage, to providea semiconductor device or the like with low power consumption, or toprovide a display device or the like that can perform accurate display.Note that the description of these objects does not exclude anotherobject. In addition, one embodiment of the present invention does notnecessarily achieve all the objects listed above.

In order to achieve any of the above objects, a circuit is formed usinga transistor including an oxide semiconductor (OS), particularly a MOStransistor including an oxide semiconductor. The oxide semiconductor isa substantially intrinsic semiconductor. For that reason, the off-statecurrent is extremely low.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a pixel portion including a plurality of pixels,and a precharge circuit. The precharge circuit includes a plurality offirst transistors. Each of the plurality of pixels includes a secondtransistor and a liquid crystal element. Each of the first transistorsand the second transistors includes an oxide semiconductor.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a pixel portion including a plurality of pixels,and an inspection circuit. The inspection circuit includes a pluralityof first transistors. Each of the plurality of pixels includes a secondtransistor and a liquid crystal element. Each of the first transistorsand the second transistors includes an oxide semiconductor.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a pixel portion including a plurality of pixels,and a circuit. The circuit includes a plurality of first transistors.Each of the plurality of pixels includes a second transistor and aliquid crystal element. Gates of the plurality of first transistors areelectrically connected to each other. First terminals of the pluralityof first transistors are electrically connected to each other. All ofsecond terminals of the plurality of first transistors are electricallyconnected to the pixel portion. The gates of the plurality of firsttransistors are in a floating state. Each of the first transistors andthe second transistors includes an oxide semiconductor.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a pixel portion including a plurality of pixels,and a circuit. The circuit includes a plurality of first transistors.Each of the plurality of pixels includes a second transistor and aliquid crystal element. Gates of the plurality of first transistors areelectrically connected to each other. First terminals of the pluralityof first transistors are electrically connected to each other. All ofsecond terminals of the plurality of first transistors are electricallyconnected to the pixel portion. The first terminals of the plurality offirst transistors are in a floating state. Each of the first transistorsand the second transistors includes an oxide semiconductor.

In any of the above liquid crystal display devices, the oxidesemiconductor included in the first transistors and the secondtransistors may be an intrinsic semiconductor.

In the invention disclosed herein, a circuit is formed using atransistor that includes an oxide semiconductor and has a low off-statecurrent; thus, unnecessary current can be prevented from being leakedinto the circuit. For that reason, the circuit is more likely to operatenormally. As a result, accurate display can be performed in a displaydevice that includes a circuit having a transistor including an oxidesemiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram illustrating a semiconductor device;

FIG. 2 is a circuit diagram illustrating a semiconductor device;

FIG. 3 is a circuit diagram illustrating a semiconductor device;

FIG. 4 is a circuit diagram illustrating a semiconductor device;

FIG. 5 is a circuit diagram illustrating a semiconductor device;

FIGS. 6A and 6B are timing charts each illustrating a waveform of asignal in a semiconductor device;

FIG. 7 is a timing chart illustrating waveforms of signals in asemiconductor device;

FIGS. 8A and 8B are circuit diagrams each illustrating a semiconductordevice;

FIGS. 9A and 9B are circuit diagrams each illustrating a semiconductordevice;

FIGS. 10A and 10B are circuit diagrams each illustrating a semiconductordevice;

FIGS. 11A and 11B are circuit diagrams each illustrating a semiconductordevice;

FIGS. 12A to 12D are cross-sectional views each illustrating asemiconductor device;

FIG. 13A is a circuit diagram illustrating a display device, and FIG.13B is a cross-sectional view illustrating a display device;

FIGS. 14A to 14C are cross-sectional views each illustrating a displaydevice;

FIGS. 15A to 15C are timing charts each illustrating a waveform in adisplay device;

FIGS. 16A to 16H each illustrate an electronic device; and

FIGS. 17A to 17H each illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to the accompanyingdrawings. Note that in structures described below, the same portions orportions having similar functions are denoted by the same referencenumerals, and description thereof is not repeated.

Embodiment 1

In this embodiment, an example of a device (a semiconductor device, adisplay device, or a light-emitting device) having a transistorincluding an oxide semiconductor, particularly a transistor including anoxide semiconductor in an active layer will be described with referenceto drawings. The off-state current of a transistor including an oxidesemiconductor is low; therefore, defects caused due to off-state currentcan be reduced in a semiconductor device or the like including an oxidesemiconductor. Moreover, a transistor including an oxide semiconductorhas high withstand voltage. For that reason, even when high voltage isapplied to the transistor, the transistor can operate normally and theoff-state current can be low; thus, defects caused due to off-statecurrent can be reduced.

FIG. 1 illustrates an example of a structure of a device in thisembodiment. The device according to one example of this embodimentincludes a pixel portion 101 and a circuit 111.

A plurality of pixels are arranged in matrix in the pixel portion 101.Here, the expression “pixels are arranged (provided) in matrix” alsoincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in thelongitudinal direction or the lateral direction. For example, a pixel102 a and a pixel 102 b are aligned in the lateral direction. Similarly,a pixel 102 c and a pixel 102 d are aligned in the lateral direction.Moreover, the pixel 102 a and the pixel 102 c are aligned in thelongitudinal direction. Similarly, the pixel 102 b and the pixel 102 dare aligned in the longitudinal direction. The pixels are connected toeach other with wirings. Pixels arranged in the longitudinal directionare connected with a wiring extended in the vertical direction. Pixelsarranged in the lateral direction are connected with a wiring extendedin the horizontal direction. For example, the pixel 102 a and the pixel102 b are connected with a wiring 104 a. Similarly, the pixel 102 c andthe pixel 102 d are connected with a wiring 104 b. Furthermore, thepixel 102 a and the pixel 102 c are connected with a wiring 103 a.Similarly, the pixel 102 b and the pixel 102 d are connected with awiring 103 b. Note that pixels can be connected with another wiring, forexample, a wiring to which all the pixels are connected (e.g., a commonwiring or a power supply line). Further, pixels other than the pixels102 a to 102 d are arranged and connected in a similar manner.

Here, the wirings provided in the horizontal direction, such as thewirings 104 a and 104 b, are sometimes connected to gates of transistorsincluded in pixels and thus can have a function of a gate signal line (agate wiring, a gate line, or the like). Alternatively, the wiringsprovided in the horizontal direction, such as the wirings 104 a and 104b, are supplied with a selection signal per row and the signal isscanned in some cases. Thus, the wirings provided in the horizontaldirection, such as the wirings 104 a and 104 b, can have a function of ascan signal line (a scan wiring, a scan line, or the like).

The wirings provided in the vertical direction, such as the wiring 103a, the wiring 103 b, a wiring 103 c, a wiring 103 d, a wiring 103 e, awiring 103 f, a wiring 103 g, a wiring 103 h, a wiring 103 i, a wiring103 j, a wiring 103 k, and a wiring 103L (the wirings 103 a to 103L),are sometimes connected to sources or drains of transistors included inpixels and thus can have a function of a source signal line (a sourcewiring, a source line, or the like). Alternatively, the wirings providedin the vertical direction, such as the wirings 103 a to 103L, aresometimes supplied with a data signal, a video signal, a source signal,or the like and thus can have a function of a data signal line (a datawiring, a data line, or the like).

The circuit 111 is connected to the pixel portion 101 or the pixelsthrough the wirings provided in the vertical direction, such as thewirings 103 a to 103L. The circuit 111 can have a variety of functions.

FIG. 2 illustrates an example of a circuit configuration showing thedetails of the circuit 111. For example, the circuit 111 can have afunction of a precharge circuit or a function of an inspection circuit.Alternatively, the circuit 111 can have both a function of a prechargecircuit and a function of an inspection circuit. Note that oneembodiment of the present invention is not limited to these examples.

The circuit 111 includes a transistor 201 a, a transistor 201 b, atransistor 201 c, a transistor 201 d, a transistor 201 e, a transistor201 f, a transistor 201 g, a transistor 201 h, a transistor 201 i, atransistor 201 j, a transistor 201 k, and a transistor 201L (thetransistors 201 a to 201L). The wirings 103 a to 103L are connected to awiring 202 through the transistors 201 a to 201L, respectively.Therefore, either sources or drains of the transistors 201 a to 201L areconnected to the wiring 202, and the rest of the sources and drains ofthe transistors 201 a to 201L are connected to the respective wirings103 a to 103L. A wiring 203 is connected to gates of the transistors 201a to 201L. That is, the gates of the transistors 201 a to 201L areconnected to each other.

Note that in this specification and the like, a transistor is an elementhaving at least three terminals of a gate, a drain, and a source. Thetransistor has a channel region between the drain (a drain terminal, adrain region, or a drain electrode) and the source (a source terminal, asource region, or a source electrode), and current can flow through thedrain, the channel region, and the source.

Note that in this specification and the like, a transistor with amulti-gate structure having two or more gate electrodes can be used, forexample. With the multi-gate structure, a structure where a plurality oftransistors are connected in series is provided because channel regionsare connected in series. Thus, with the multi-gate structure, the amountof off-state current can be reduced and the withstand voltage of thetransistor can be increased (the reliability can be improved).Alternatively, with the multi-gate structure, drain-source current doesnot change much even if drain-source voltage changes when the transistoroperates in a saturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having an extremely large resistance can be realized.Accordingly, a differential circuit, a current mirror circuit, or thelike having excellent properties can be realized.

Note that a transistor with a structure where gate electrodes are formedabove and below a channel can be used, for example. With the structurewhere the gate electrodes are formed above and below the channel, acircuit structure where a plurality of transistors are connected inparallel is provided. Thus, a channel region is increased, so that theamount of current can be increased. Alternatively, with the structurewhere the gate electrodes are formed above and below the channel, adepletion layer is easily formed; thus, the subthreshold swing value (Svalue) can be improved.

The circuit 111 is not limited to having the configuration in FIG. 2 andcan have a variety of configuration. FIG. 3 illustrates another exampleof the circuit 111. The description of FIG. 1 and FIG. 2 can be appliedto FIG. 3.

FIG. 3 illustrates an example of the case where a plurality of wirings202 are provided in FIG. 2. FIG. 3 shows the case where three wirings202 are provided; however, one embodiment of the present invention isnot limited to this example, and two wirings 202 or four or more wirings202 can be provided. The wirings 103 a to 103L are connected to one of awiring 202 a, a wiring 202 b, and a wiring 202 c through the respectivetransistors 201 a to 201L. Therefore, either sources or drains of thetransistors 201 a to 201L are connected to one of the wirings 202 a, 202b, and 202 c, and the rest of the sources and drains of the transistors201 a to 201L are connected to the respective wirings 103 a to 103L. Thewiring 203 is connected to gates of the transistors 201 a to 201L. Thatis, the gates of the transistors 201 a to 201L are connected to eachother.

In the case of performing color display, each color of pixels performingdisplay can correspond to one of the wirings 202 a, 202 b, and 202 c.For example, a wiring that is connected to the wiring 202 a through atransistor is connected to red pixels. Therefore, the wiring 202 a isconnected to the wirings 103 a, 103 d, 103 g, and 103 j through therespective transistors. Similarly, a wiring that is connected to thewiring 202 b through a transistor is connected to blue pixels.Therefore, the wiring 202 b is connected to the wirings 103 b, 103 e,103 h, and 103 k through the respective transistors. Similarly, a wiringthat is connected to the wiring 202 c through a transistor is connectedto green pixels. Therefore, the wiring 202 c is connected to the wirings103 c, 103 f, 103 i, and 103L through the respective transistors. Notethat the colors are not limited to red, blue, and green; for example,four colors (RGB and white) can be used or a plurality of slightlydifferent reds (or blues or greens) can be used. Accordingly, colors canbe controlled.

Next, FIG. 4 illustrates another example of the configuration of thecircuit 111. The description of FIG. 1, FIG. 2, and FIG. 3 can beapplied to FIG. 4.

FIG. 4 illustrates an example of the case where a plurality of wirings203 are provided in FIG. 2. FIG. 4 shows the case where three wirings203 are provided; however, one embodiment of the present invention isnot limited to this example, and two wirings 203 or four or more wirings203 can be provided. The wirings 103 a to 103L are connected to thewiring 202 through the transistors 201 a to 201L, respectively.Therefore, either sources or drains of the transistors 201 a to 201L areconnected to the wiring 202, and the rest of the sources and drains ofthe transistors 201 a to 201L are connected to the respective wirings103 a to 103L. Gates of the transistors 201 a to 201L are connected toone of a wiring 203 a, a wiring 203 b, and a wiring 203 c. That is, thegates of some of the transistors 201 a to 201L (e.g., the transistors201 a, 201 d, 201 g, and 201 j) are connected to each other and to thewiring 203 a. Similarly, the gates of some of the transistors 201 a to201L (e.g., the transistors 201 b, 201 e, 201 h, and 201 k) areconnected to each other and to the wiring 203 b. Similarly, the gates ofthe others (e.g., the transistors 201 c, 201 f, 201 i, and 201L) areconnected to each other and to the wiring 203 c.

In the case of performing color display, each color of pixels performingdisplay can correspond to one of the wirings 203 a, 203 b, and 203 c.For example, a wiring that is connected to the wiring 202 through atransistor whose gate is connected to the wiring 203 a is connected tored pixels. Therefore, the wiring 202 is connected to the wirings 103 a,103 d, 103 g, and 103 j through the respective transistors each havingthe gate connected to the wiring 203 a. Similarly, a wiring that isconnected to the wiring 202 through a transistor whose gate is connectedto the wiring 203 b is connected to blue pixels. Therefore, the wiring202 is connected to the wirings 103 b, 103 e, 103 h, and 103 k throughthe respective transistors each having the gate connected to the wiring203 b. Similarly, a wiring that is connected to the wiring 202 through atransistor whose gate is connected to the wiring 203 c is connected togreen pixels. Therefore, the wiring 202 is connected to the wirings 103c, 103 f, 103 i, and 103L through the respective transistors each havingthe gate connected to the wiring 203 c. Note that the colors are notlimited to red, blue, and green; for example, four colors (RGB andwhite) can be used or a plurality of slightly different reds (or bluesor greens) can be used. Accordingly, colors can be controlled.

Next, FIG. 5 illustrates another example of the configuration of thecircuit 111. The description of FIG. 1, FIG. 2, FIG. 3, and FIG. 4 canbe applied to FIG. 5.

FIG. 5 illustrates an example of the case where a plurality of wirings202 and a plurality of wirings 203 are provided in FIG. 2. That is, FIG.5 corresponds to the case where a plurality of wirings 203 are providedin FIG. 3 and the case where a plurality of wirings 202 are provided inFIG. 4. FIG. 5 shows the case where three wirings 202 and three wirings203 are provided; however, one embodiment of the present invention isnot limited to this example, and the number of the wiring 202 and/or thewiring 203 can be two or four or more. The wirings 103 a to 103L areconnected to one of the wirings 202 a, 202 b, and 202 c through therespective transistors 201 a to 201L. Therefore, either sources ordrains of the transistors 201 a to 201L are connected to one of thewirings 202 a, 202 b, and 202 c, and the rest of the sources and drainsof the transistors 201 a to 201L are connected to the respective wirings103 a to 103L. Gates of the transistors 201 a to 201L are connected toone of the wirings 203 a, 203 b, and 203 c. That is, the gates of someof the transistors 201 a to 201L (e.g., the transistors 201 a, 201 d,201 g, and 201 j) are connected to each other and to the wiring 203 a.Similarly, the gates of some of the transistors 201 a to 201L (e.g., thetransistors 201 b, 201 e, 201 h, and 201 k) are connected to each otherand to the wiring 203 b. Similarly, the gates of the others (e.g., thetransistors 201 c, 201 f, 201 i, and 201L) are connected to each otherand to the wiring 203 c.

In the case of performing color display, as in FIG. 2, FIG. 3, and FIG.4, each color of pixels performing display can correspond to one of thewirings 203 a, 203 b, and 203 c or one of the wirings 202 a, 202 b, and202 c. Accordingly, colors can be controlled.

Embodiment 2

In this embodiment, an example of the operation of the circuit 111illustrated in FIG. 2, FIG. 3, FIG. 4, and FIG. 5 will be described.

The circuit 111 can have a variety of functions depending on anoperation method. For example, the circuit 111 can have a function of aninspection circuit and/or a function of a precharge circuit. Note thatone embodiment of the present invention is not limited to this example,and the circuit 111 can have another function.

First, an operation method in the case where the circuit 111 is operatedas a precharge circuit will be described. FIGS. 6A and 6B show waveformsin the case where precharge is performed. FIG. 6A shows a waveform of asignal supplied to a wiring in a given row, for example, the wiring 104a. A signal 901 has a waveform with a cycle 902. The cycle 902 oftencorresponds to one frame period or one subframe period, for example. Inthe cycle 902, the signal 901 is at H level only in a period 903 and isat L level in the other periods. The period 903 often corresponds to onegate selection period or 1H period, for example. FIG. 6B shows detailedwaveforms in one gate selection period. One gate selection period can beconsidered to be divided into a former period 904 and a latter period905. In the former period 904, a predetermined voltage can be suppliedto the wirings 103 a to 103L in advance. Such an operation for supplyinga predetermined voltage in advance corresponds to precharge. That is,the former period 904 can be referred to as a precharge period. In thelatter period 905, a video signal is supplied. That is, the latterperiod 905 can be referred to as a signal input period. As an example, asignal 908 represents a signal supplied to any one of the wirings 103 ato 103L. The potential of the signal 908 becomes approximately equal toa potential 906 in the former period 904. Here, the potential 906 can bea reference potential, for example, a potential that is approximatelyequal to a common potential or a counter potential supplied to a displayelement (e.g., a liquid crystal element). Alternatively, the potential906 preferably lies nearly in the middle of the amplitude of thepotential of a video signal. In the latter period 905, the potential ofthe signal 908 becomes a potential corresponding to a video signal.Accordingly, the potential of the signal 908 in the latter period 905varies as appropriate depending on the polarity of a video signal or asignal supplied to a liquid crystal element, for example. By thussupplying a predetermined voltage to the wirings 103 a to 103L inadvance, a signal can be supplied rapidly. Thus, the speed of writingsignals is increased, leading to a reduction in the time for writingsignals.

In the circuit in FIG. 2, a signal like a signal 907 is supplied to thewiring 203. In the former period 904, the signal 907 is a voltage withwhich the transistors 201 a to 201L are turned on. Moreover, thepotential 906 is supplied to the wiring 202. Thus, the wirings 103 a to103L can be precharged in the former period 904.

In this case, when the potential of each wiring is controlled by atransistor including an oxide semiconductor, adverse effects of theoff-state current of the transistor can be reduced; thus, leakage ofvoltage and noise can be reduced, and accurate voltages can be suppliedto wirings. Therefore, the use of the transistor including an oxidesemiconductor can realize accurate display.

As described above, the transistors 201 a to 201L have a function ofbeing turned on or off, controlled with a gate signal. Accordingly, thetransistors 201 a to 201L can have a switch function.

The circuit in FIG. 3 includes a plurality of wirings 202 a to 202 c.For that reason, in the former period 904, different potentials can besupplied to the wirings 103 a to 103L. Therefore, for example,potentials different depending on colors can be supplied to the wirings103 a to 103L in the former period 904. Accordingly, the circuit canoperate optimally to express various colors.

The circuit in FIG. 4 includes a plurality of wirings 203 a to 203 c.For that reason, when the former period 904 is further divided into aplurality of periods, only one of the transistors 201 a to 201L can beturned on. FIG. 7 shows waveforms of signals supplied to the wirings 203a to 203 c in that case. A signal 907 a represents a signal supplied tothe wiring 203 a. A signal 907 b represents a signal supplied to thewiring 203 b. A signal 907 c represents a signal supplied to the wiring203 c. The signal 907 a is at H level only in the first third of theformer period 904. The signal 907 b is at H level only in the middlethird of the former period 904. The signal 907 c is at H level only inthe final third of the former period 904. By thus dividing the formerperiod 904 into a plurality of periods so that potentials supplied tothe wiring 202 are made different in the plurality of periods,potentials supplied to the wirings 103 a to 103L can be made to vary.Therefore, in the former period 904, potentials different depending oncolors can be supplied to the wirings 103 a to 103L. Accordingly, thecircuit can operate optimally in accordance with colors.

The circuit in FIG. 5 includes a plurality of wirings 202 a to 202 c.For that reason, in the former period 904, different potentials can besupplied to the wirings 103 a to 103L. Furthermore, the circuit in FIG.5 includes a plurality of wirings 203 a to 203 c. For that reason, whenthe former period 904 is divided into a plurality of periods asillustrated in FIG. 7, only one of the transistors 201 a to 201L can beturned on. By thus dividing the former period 904 into a plurality ofperiods so that potentials supplied to the wirings 202 a to 202 c aremade different in the plurality of periods, potentials supplied to thewirings 103 a to 103L can be made to vary. Therefore, in the formerperiod 904, potentials different depending on colors can be supplied tothe wirings 103 a to 103L. Accordingly, the circuit can operateoptimally in accordance with colors.

Next, an operation method in the case where the circuit 111 is operatedas an inspection circuit will be described. First, the case of thecircuit 111 in FIG. 2 is described.

First, a signal is supplied to a pixel by a given means. For example, asignal is supplied to the wirings 103 a to 103L from a signal linedriver circuit connected to the wirings 103 a to 103L. Preferably, asignal is supplied to one of the wirings 103 a to 103L. Alternatively,the transistors 201 a to 201L are turned on to supply a signal to thewirings 103 a to 103L, preferably one of the wirings 103 a to 103L,through the wiring 202. Further alternatively, a probe for supplying asignal comes into contact with the wirings 103 a to 103L, preferably oneof the wirings 103 a to 103L, to supply a signal. Then, an H-levelsignal is supplied to any one of gate lines such as the wirings 104 aand 104 b. Thus, the signal is supplied into a pixel when a transistorin the pixel operates normally. After that, an L-level signal issupplied to that gate line. As a result, the signal is held in thepixel.

Next, an H-level signal is supplied to the gate line, so that the signalheld in the pixel is supplied to the wirings 103 a to 103L. At thistime, an H-level signal is supplied to the wiring 203 to turn on thetransistors 201 a to 201L. Thus, the signal held in the pixel can beread through the wiring 202.

In this case, if the transistor in the pixel is defective, the signalcannot be properly read. Therefore, whether the transistor normallyoperates or not can be inspected depending on whether the signal isproperly read.

In the case where a signal is supplied to one of the wirings 103 a to103L, a signal is then supplied to another one of the wirings 103 a to103L and a similar operation is repeated. Further, this operation isperformed on all the gate lines. Thus, all the pixels can be inspected.In the case where one gate line is being selected, when a signal issupplied to only one of a plurality of pixels connected to the gate lineand reading is performed on the pixel, inspection can be accuratelyperformed on a pixel-by-pixel basis. On the other hand, when a signal issupplied to some or all of a plurality of pixels and reading isperformed on the pixels, the pixels can be roughly inspected.

In FIG. 3, a plurality of wirings 202 a to 202 c are provided. For thatreason, signals held in a plurality of pixels can be read at the sametime through the wirings 202 a to 202 c. Three wirings are provided inthe case of FIG. 3; therefore, signals can be supplied to three pixelsat the same time, and the signals in three pixels can be read at thesame time. Thus, rapid inspection can be realized. As a result, theduration of a manufacturing process of a semiconductor device or thelike can be reduced, and costs can be reduced.

In FIG. 4, a plurality of wirings 203 a to 203 c are provided. For thatreason, signals held in pixels can be sequentially read through thewiring 202 in such a manner that signals supplied to the wirings 203 ato 203 c are sequentially set at H level. Accordingly, signals can besupplied to a plurality of pixels at the same time and sequentially readon a pixel-by-pixel basis. Thus, rapid inspection can be realized. As aresult, the duration of a manufacturing process of a semiconductordevice or the like can be reduced, and costs can be reduced.

In FIG. 5, a plurality of wirings 202 a to 202 c are provided. For thatreason, signals held in a plurality of pixels can be read at the sametime through the wirings 202 a to 202 c. Moreover, a plurality ofwirings 203 a to 203 c are provided. For that reason, signals held inpixels can be sequentially read through the wiring 202 in such a mannerthat signals supplied to the wirings 203 a to 203 c are sequentially setat H level. Thus, rapid inspection can be realized. As a result, theduration of a manufacturing process of a semiconductor device or thelike can be reduced, and costs can be reduced.

Such inspection is performed as part of a process for manufacturing asemiconductor device or the like. Therefore, an inspection circuit isnot operated after the semiconductor device or the like is assembled andshipped.

If the off-state current of the transistors 201 a to 201L is large,current is leaked, so that noise enters the wirings 103 a to 103L. Inview of the above, by using a transistor including an oxidesemiconductor as the transistors 201 a to 201L, the off-state currentcan be reduced and noise can be reduced.

As described above, by changing an operation method, the circuit 111 canrealize a variety of functions. Accordingly, the circuit 111 can haveboth a function of a precharge circuit and a function of an inspectioncircuit. When the circuit 111 has both of the functions, a larger numberof functions can be realized with a smaller number of circuits,resulting in lower cost, lower power consumption, and the like.

Embodiment 3

In this embodiment, examples of peripheral circuits of the circuit 111will be described.

FIGS. 8A and 8B each illustrate the circuit 111, the pixel portion 101,and an example of peripheral circuits. The circuit 111 and the pixelportion 101 are placed over a substrate 511. In other words, atransistor and a wiring included in the circuit 111 are formed, etched,and patterned at the same time as a transistor and a wiring included inthe pixel portion 101. That is, the circuit 111 and the pixel portion101 are formed at the same time over one substrate through the sameprocess. Thus, materials of the transistor and the wiring in the circuit111 are the same as those of the transistor and the wiring in the pixelportion 101. For that reason, when the transistor in the pixel portion101 includes an oxide semiconductor, the transistor in the circuit 111also includes the oxide semiconductor.

Note that in this specification and the like, a transistor can be formedusing a variety of substrates. There is no particular limitation on thekind of a substrate. Examples of the substrate are a semiconductorsubstrate (e.g., a single crystal substrate and a silicon substrate), anSOI substrate, a glass substrate, a quartz substrate, and a flexiblesubstrate. For a flexible substrate, a flexible synthetic resin such asplastics typified by polyethylene terephthalate (PET) and polyethylenenaphthalate (PEN), or acrylic can be used, for example.

Note that a transistor may be formed using one substrate, and then thetransistor may be transferred to another substrate.

Note that all the circuits that are necessary to realize a desiredfunction can be formed using one substrate. Thus, costs can be reducedby reduction in the number of components, or the reliability can beimproved by reduction in the number of connections to circuitcomponents.

In FIG. 8A, a circuit 501, a circuit 502, a circuit 503, and a circuit504 are formed over substrates different from the substrate 511. Forexample, the circuit 501, the circuit 502, the circuit 503, or thecircuit 504 includes an IC chip formed using a single crystal substrateor an SOI substrate. Note that the circuit 501, the circuit 502, thecircuit 503, and/or the circuit 504 may be provided over the substrate511 by COG (chip on glass).

Here, the circuit 501 has a function of controlling the circuit 111,supplying a signal and a voltage to the circuit 111, or reading a signalfrom the circuit 111, for example. The 501 has a function correspondingto the function of the circuit 111. For example, when the circuit 111has a function of a precharge circuit, the circuit 501 has a function ofcontrolling a precharge operation. Similarly, when the circuit 111 has afunction of an inspection circuit, the circuit 501 has a function ofcontrolling an inspection operation. Similarly, when the circuit 111 hasa function of a precharge circuit and a function of an inspectioncircuit, the circuit 501 has a function of controlling a prechargeoperation and a function of controlling an inspection operation.

The circuit 502 has a function of supplying a signal to the wirings 104a and 104 b and the like; therefore, the circuit 502 can have a functionof a gate line driver circuit (a scan driver). The circuit 503 has afunction of supplying a signal to the wirings 103 a to 103L; therefore,the circuit 503 can have a function of a signal line driver circuit (adata driver). The circuit 504 has a function of controlling the circuit502 or the circuit 503. Therefore, the circuit 504 can have a functionof a controller, a pulse generator circuit, a clock signal generatorcircuit, a common voltage generator circuit, a timing generator circuit,an image processing circuit, a power supply circuit, or the like.

FIG. 8B illustrates the case where the circuit 501 is not provided inthe structure of FIG. 8A. In the case where the circuit 111 functions asan inspection circuit, the circuit 501 is necessary when the circuit 111performs inspection but not necessary after the inspection. Therefore,the circuit 501 can be provided as illustrated in FIG. 8A at the time ofinspection, and the circuit 501 can be removed after the inspection isfinished. When the circuit 501 is removed as shown in FIG. 8B, thewiring 202 and the wiring 203 are each in a floating state.

Note that the circuits 501, 502, 503, and 504 are provided oversubstrates different from the substrate 511 in FIGS. 8A and 8B; however,one embodiment of the present invention is not limited to theseexamples. For example, some of these circuits can be provided over thesubstrate 511. As an example, FIGS. 9A and 9B each illustrate an exampleof the case where the circuit 502 is provided over the substrate 511.Like FIG. 8A, FIG. 9A illustrates a structure including the circuit 501.Like FIG. 8B, FIG. 9B illustrates a structure without the circuit 501.That is, in FIGS. 9A and 9B, the circuit 111, the pixel portion 101, andthe circuit 502 are formed over one substrate through the same process.For that reason, when the transistor in the pixel portion 101 includesan oxide semiconductor, transistors in the circuits 111 and 502 alsoinclude the oxide semiconductor. When the circuit 501 is removed asshown in FIG. 8B, the wiring 202 and the wiring 203 are each in afloating state.

By thus forming the circuit 502 over the same substrate as the pixelportion 101, costs can be reduced.

As an example different from FIGS. 9A and 9B, the circuit 503 or part ofthe circuit 503 can be provided over the substrate 511. As an example ofpart of the circuit 503, an analog switch (a transfer gate) can beconnected to the wirings 103 a to 103L. Similarly, the circuit 504 orpart of the circuit 504 can be provided over the substrate 511.

Embodiment 4

In this embodiment, examples of a pixel included in the pixel portion101 will be described.

FIG. 10A illustrates an example of a circuit of the pixel 102 a. A gateof a transistor 801 is connected to the wiring 104 a. A first terminalof the transistor 801 is connected to the wiring 103 a. A secondterminal of the transistor 801 is connected to a first terminal of adisplay element 802. A second terminal of the display element 802 isconnected to a wiring 803. A first terminal of a capacitor 804 isconnected to the second terminal of the transistor 801. A secondterminal of the capacitor 804 is connected to a wiring 805.

Here, the wirings 803 can be connected to each other in all the pixels.Moreover, a predetermined voltage is supplied to the wiring 803. Forthat reason, the wiring 803 can have a function of a common wiring, acounter electrode, or the like.

The wirings 805 can be connected to each other in other pixels, forexample, horizontal pixels. Moreover, a predetermined voltage issupplied to the wiring 805. For that reason, the wiring 805 can have afunction of a common wiring, a capacitor wiring, or the like.

The transistor 801 can have a function of selecting whether or not tosupply a signal to the display element 802 or the capacitor 804.Accordingly, the transistor 801 can have a switch function.Alternatively, the transistor 801 can have a function of a switchingtransistor (a transistor for switching) or a selection transistor.

Note that the capacitor 804 can be omitted so that the aperture ratio isincreased. FIG. 10B illustrates a circuit diagram in that case. By usinga transistor with low off-state current, for example, a transistorincluding an oxide semiconductor as the transistor 801, a voltageapplied to the display element 802 can be retained even when thecapacitor 804 is omitted.

Note that examples of the pixel circuit are not limited to the circuitsillustrated in FIGS. 10A and 10B. FIG. 11A illustrates another exampleof a pixel circuit. A gate of a transistor 1301 is connected to thewiring 104 a. A first terminal of the transistor 1301 is connected tothe wiring 103 a. A second terminal of the transistor 1301 is connectedto a gate of a transistor 1311. A first terminal of the transistor 1311is connected to a wiring 1305. A second terminal of the transistor 1311is connected to a first terminal of the display element 802. A secondterminal of the display element 802 is connected to a wiring 1303. Afirst terminal of a capacitor 1304 is connected to the second terminalof the transistor 1301. A second terminal of the capacitor 1304 isconnected to the second terminal of the transistor 1311.

Note that a wiring or a terminal to which the second terminal of thecapacitor 1304 is connected is not limited to the example in FIG. 11A;the second terminal of the capacitor 1304 can be connected to anotherwiring, for example, the wiring 1305.

Here, the wirings 1303 can be connected to each other in all the pixels.Moreover, a predetermined voltage is supplied to the wiring 1303. Forthat reason, the wiring 1303 can have a function of a common wiring, acounter electrode, a cathode wiring, a cathode line, or the like.

The wirings 1305 can be connected to each other in other pixels, forexample, horizontal pixels or vertical pixels. Moreover, a predeterminedvoltage is supplied to the wiring 1305. For that reason, the wiring 1305can have a function of a common wiring, a capacitor wiring, a powersupply line, a current supply line, an anode line, or the like.

The transistor 1301 can have a function of selecting whether or not tosupply a signal to the transistor 1311 or the capacitor 1304.Accordingly, the transistor 1301 can have a switch function.Alternatively, the transistor 1301 can have a function of a switchingtransistor or a selection transistor.

The transistor 1311 can have a switch function of selecting whether ornot to supply a signal, a voltage, or a current to the display element802; and a function of a current source for controlling the magnitude ofthe signal, voltage, or current. Alternatively, the transistor 1311 canhave a function of a driving transistor.

Note that the capacitor 1304 can be omitted so that the aperture ratiois increased. FIG. 11B illustrates a circuit diagram in that case. Byusing a transistor with low off-state current, for example, a transistorincluding an oxide semiconductor as the transistors 1301 and 1311, avoltage applied to the display element 802 can be retained even when thecapacitor 1304 is omitted.

A variety of elements can be used as the display element 802, and thedisplay element 802 can function as a light-emitting element in somecases. FIG. 11B illustrates a circuit diagram in the case where alight-emitting element 802 a is used as a display element.

Examples of display elements are elements including a display mediumwhose contrast, luminance, reflectance, transmittance, or the likechanges by electromagnetic action, such as an EL (electroluminescence)element (e.g., an EL element containing organic and inorganic materials,an organic EL element, and an inorganic EL element), an LED (e.g., awhite LED, a red LED, a green LED, and a blue LED), a liquid crystalelement, electronic ink, an electrophoretic element, and a carbonnanotube. An example of a display device including liquid crystalelements is a liquid crystal display. An example of a display deviceincluding electronic ink or electrophoretic elements is electronicpaper.

An example of an EL element is an element including an anode, a cathode,and an EL layer placed between the anode and the cathode. Examples of anEL layer are a layer utilizing light emission (fluorescence) from asinglet exciton, a layer utilizing light emission (phosphorescence) froma triplet exciton, a layer utilizing light emission (fluorescence) froma singlet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed using an organic material, a layer formed usingan inorganic material, a layer formed using an organic material and aninorganic material, a layer including a high-molecular material, a layerincluding a low-molecular material, and a layer including ahigh-molecular material and a low-molecular material. Note that varioustypes of EL elements can be used without limitation to the above.

An example of a liquid crystal element is an element that controlstransmission and non-transmission of light by optical modulation actionof liquid crystals. The element can include a pair of electrodes and aliquid crystal layer. The optical modulation action of liquid crystalsis controlled by an electric field applied to the liquid crystals(including a lateral electric field, a vertical electric field, and adiagonal electric field). Moreover, the following methods can be usedfor driving the liquid crystals, for example: a TN (twisted nematic)mode, an STN (super twisted nematic) mode, an IPS (in-plane-switching)mode, an FFS (fringe field switching) mode, an MVA (multi-domainvertical alignment) mode, a PVA (patterned vertical alignment) mode, anASV (advanced super view) mode, an ASM (axially symmetric alignedmicrocell) mode, an OCB (optically compensated birefringence) mode, anECB (electrically controlled birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (anti-ferroelectric liquid crystal) mode,a PDLC (polymer dispersed liquid crystal) mode, a PNLC (polymer networkliquid crystal) mode, a guest-host mode, and a blue phase mode. Notethat various liquid crystal elements and driving methods can be usedwithout limitation to the above.

Since there are few limitations on the layout of the transistorsincluded in the circuit 111, the channel width (or the gate width) W ofthe transistors included in the circuit 111 (e.g., the transistors 201 ato 201L) can be larger than the channel width (or the gate width) W ofthe transistor included in the pixel (e.g., the transistors 801, 1301,and 1311). In particular, a transistor including an oxide semiconductorhas a low off-state current; therefore, an increase in the channel width(or the gate width) W causes fewer adverse effects. Further, when thechannel width (or the gate width) W of the transistors included in thecircuit 111 is increased, precharge and inspection can be rapidlyperformed. Similarly, the channel length (or the gate length) L of thetransistors included in the circuit 111 (e.g., the transistors 201 a to201L) can be larger than the channel length (or the gate length) L ofthe transistor included in the pixel (e.g., the transistors 801, 1301,and 1311).

Embodiment 5

In this embodiment, a transistor in which a channel formation region isformed using an oxide semiconductor is used as each of the transistors201 a to 201L, the transistor 801, the transistor 1301, the transistor1311, and the like.

Examples of oxide semiconductors are an In—Sn—Ga—Zn—O-based oxidesemiconductor which is an oxide of four metal elements; anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, and an In—Mg—O-based oxide semiconductor which are oxidesof two metal elements; an In—O-based oxide semiconductor, a Sn—O-basedoxide semiconductor, and a Zn—O-based oxide semiconductor. Further, theabove-described metal oxide semiconductor may contain SiO₂.

As the oxide semiconductor, a material expressed by InMO₃(ZnO)_(m) (m>0)can be used. Here, M represents one or more metal elements selected fromGa, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, orGa and Co. An oxide semiconductor whose composition formula isrepresented by InMO₃(ZnO)_(m) (m>0) where Ga is included as M isreferred to as an In—Ga—Zn—O oxide semiconductor, and a thin filmthereof is also referred to as an In—Ga—Zn—O-based film.

One embodiment of a method for manufacturing a transistor in which achannel formation region is formed using an oxide semiconductor will bedescribed with reference to FIGS. 12A to 12D.

FIGS. 12A to 12D illustrates an example of a cross-sectional structureof a transistor. A transistor 410 illustrated in FIGS. 12A to 12D has abottom-gate structure called a channel-etched structure.

Although a single-gate transistor is illustrated in FIGS. 12A to 12D, amulti-gate transistor including a plurality of channel formation regionscan be formed as needed.

Steps for manufacturing the transistor 410 over a substrate 400 will bedescribed below with reference to FIGS. 12A to 12D.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then, a gate electrode layer 411 is formedthrough a first photolithography process.

Although there is no particular limitation on a substrate that can beused as the substrate 400 having an insulating surface, the substrateneeds to have heat resistance high enough to at least withstand heattreatment to be performed later. For example, a glass substrate made ofbarium borosilicate glass, aluminoborosilicate glass, or the like can beused. A glass substrate whose strain point is 730° C. or more ispreferably used when the temperature of the heat treatment performedlater is high.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 411. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed with a single-layer structure or alayered structure using one or more of a silicon nitride film, a siliconoxide film, a silicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 411 can be formed with a single-layer structureor a layered structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium or an alloy material containing any of these materials as itsmain component.

Then, a gate insulating layer 402 is formed over the gate electrodelayer 411.

The gate insulating layer 402 can be formed with a single-layerstructure or a layered structure using a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, or an aluminum oxide layer by a plasma CVD method, a sputteringmethod, or the like. Furthermore, a high-k material such as hafniumoxide (HfOx) or tantalum oxide (TaOx) can be used as the gate insulatinglayer. The thickness of the gate insulating layer 402 is 100 nm to 500nm. In the case of employing a layered structure, the gate insulatinglayer 402 is a stack, for example, of a first gate insulating layer witha thickness of 50 nm to 200 nm and a second gate insulating layer with athickness of 5 nm to 300 nm over the first gate insulating layer.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm or less is formed by plasma CVD as the gate insulating layer 402.

Further, as the gate insulating layer 402, a silicon oxynitride film maybe formed using a high-density plasma apparatus. Here, a high-densityplasma apparatus refers to an apparatus that can realize a plasmadensity of 1×10¹¹/cm³ or higher. For example, plasma is generated byapplying a microwave power of 3 kW 6 kW, and the insulating film isformed. The insulating film formed by the high-density plasma apparatuscan have a uniform thickness, and thus has excellent step coverage.Moreover, the thickness of a thin insulating film formed using thehigh-density plasma apparatus can be precisely controlled.

The insulating film obtained with the high-density plasma apparatus isgreatly different from an insulating film obtained with a conventionalparallel plate PCVD apparatus. The etching rate of the insulating filmobtained with the high-density plasma apparatus is lower than that ofthe insulating film obtained with a parallel plate PCVD apparatus by 10%or more or 20% or more in the case where the etching rates with the sameetchant are compared to each other. Thus, it can be said that theinsulating film obtained with the high-density plasma apparatus is adense film.

The oxide semiconductor that becomes an i-type or substantially i-typeoxide semiconductor (a purified oxide semiconductor) in a later step isextremely sensitive to the interface state or interface electric charge;therefore, the interface with the gate insulating layer is important.For that reason, the gate insulating layer (GI) that is in contact withthe purified oxide semiconductor needs to have high quality. Therefore,high-density plasma CVD with use of microwaves (2.45 GHz) is preferablyemployed since a dense and high-quality insulating film having highwithstand voltage can be formed. When the purified oxide semiconductorand the high-quality gate insulating layer are in close contact witheach other, the interface state density can be reduced and favorableinterface characteristics can be obtained. It is important that the gateinsulating layer have lower interface state density with an oxidesemiconductor and a favorable interface as well as having favorable filmquality as a gate insulating layer.

Then, an oxide semiconductor film 430 is formed to a thickness of 2 nmto 200 nm over the gate insulating layer 402. As the oxide semiconductorfilm 430, an In—Ga—Zn—O-based oxide semiconductor film, an In—Zn—O-basedoxide semiconductor film, or the like is used. In this embodiment, theoxide semiconductor film 430 is formed by a sputtering method with theuse of an In—Ga—Zn—O-based oxide semiconductor target. A cross-sectionalview at this stage is illustrated in FIG. 12A. Alternatively, the oxidesemiconductor film 430 can be formed by a sputtering method in a raregas (typically argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas (typically argon) and oxygen.

Here, film deposition is performed using a metal oxide target containingIn, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]). The depositionconditions are set as follows: the distance between the substrate 400and the target is 100 mm; the pressure is 0.2 Pa; the direct current(DC) power supply is 0.5 kW; and the atmosphere is a mixed atmosphere ofargon and oxygen (argon:oxygen=30 sccm:20 sccm and the flow rate ratioof oxygen is 40%). Note that a pulse direct current (DC) power supply ispreferably used because powder substances generated at the time ofdeposition can be reduced and the film thickness can be made uniform.The In—Ga—Zn—O-based film is formed to a thickness of 5 nm to 200 nm. Inthis embodiment, as the oxide semiconductor film, a 20-nm-thickIn—Ga—Zn—O-based film is formed by a sputtering method with the use ofan In—Ga—Zn—O-based metal oxide target. Then, the oxide semiconductorfilm 430 is processed into an island-shaped oxide semiconductor layerthrough a second photolithography process.

Next, dehydration or dehydrogenation of the oxide semiconductor layersis performed. The temperature of first heat treatment for dehydration ordehydrogenation is higher than or equal to 400° C. and lower than orequal to 750° C., preferably higher than or equal to 400° C. and lowerthan the strain point of the substrate. Here, the substrate isintroduced into an electric furnace which is one of heat treatmentapparatuses, heat treatment is performed on the oxide semiconductorlayer at 450° C. for one hour in a nitrogen atmosphere, and then, theoxide semiconductor layer is not exposed to the air so that entry ofwater and hydrogen into the oxide semiconductor layer is prevented;thus, an oxide semiconductor layer 431 is obtained (see FIG. 12B).

Note that a heat treatment apparatus is not limited to an electricfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas that does not react with an object to be processed by heattreatment, for example, nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, GRTA by which the substrate ismoved into an inert gas heated to a high temperature as high as 650° C.to 700° C., heated for several minutes, and moved out of the inert gasheated to the high temperature may be performed. With GRTA,high-temperature heat treatment for a short period of time can beachieved.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not included in the atmosphere of nitrogen, arare gas such as helium, neon, or argon, or dry air. For example, thepurity of nitrogen or a rare gas such as helium, neon, or argon, whichis introduced into the heat treatment apparatus, is preferably 6N(99.9999%) or more, more preferably 7N (99.99999%) or more (i.e.,impurity concentration is preferably 1 ppm or lower, more preferably 0.1ppm or lower).

The first heat treatment for the oxide semiconductor layer may beperformed on the oxide semiconductor film 430 before being processedinto the island-shaped oxide semiconductor layer. In that case, afterthe first heat treatment, the substrate is taken out of the heattreatment apparatus, and then the second photolithography process isperformed.

In the case where an opening portion is formed in the gate insulatinglayer 402, the step of forming the opening portion may be performedbefore or after the oxide semiconductor film 430 is subjected todehydration or dehydrogenation treatment.

Note that the etching of the oxide semiconductor film 430 is not limitedto wet etching and dry etching may also be used.

As an etching gas used for dry etching of the oxide semiconductor film430, a gas containing chlorine (e.g., chlorine (Cl₂) or boron chloride(BCl₃)) is preferably used.

As an etchant used for wet etching of the oxide semiconductor film 430,a mixed solution of phosphoric acid, acetic acid, and nitric acid;ITO07N (produced by Kanto Chemical Co., Inc.); or the like can be used.

Next, a metal conductive film is formed over the gate insulating layer402 and the oxide semiconductor layer 431. The metal conductive film maybe formed by a sputtering method or a vacuum evaporation method.Examples of a material for the metal conductive film are an elementselected from aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd), andscandium (Sc); an alloy containing any of these elements as a component;and an alloy containing any of these elements in combination. A nitridefilm of any of these elements may be used. Alternatively, one or morematerials selected from manganese (Mn), magnesium (Mg), zirconium (Zr),beryllium (Be), and yttrium (Y) may be used. The metal conductive filmmay have a single-layer structure or a layered structure of two or morelayers. For example, the metal conductive film can have a single-layerstructure of an aluminum film containing silicon; a two-layer structureof an aluminum film and a titanium film stacked thereover; or athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in this order.

When heat treatment is performed after formation of the metal conductivefilm, the metal conductive film preferably has heat resistance highenough to withstand the heat treatment.

Through a third photolithography process, a resist mask is formed overthe metal conductive film and etching is selectively performed, so thata source electrode layer 415 a and a drain electrode layer 415 b areformed. Then, the resist mask is removed (see FIG. 12C).

In this embodiment, a titanium film is used as the metal conductivefilm, an In—Ga—Zn—O-based oxide is used for the oxide semiconductorlayer 431, and an ammonia peroxide mixture (31 wt % hydrogen peroxidesolution: 28 wt % ammonia water:water=5:2:2) is used as the etchant forthe titanium film.

Note that through the third photolithography process, only part of theoxide semiconductor layer 431 is sometimes etched so that a groove (arecessed portion) is formed in the oxide semiconductor layer.

In order to reduce the number of photomasks used in the photolithographyprocesses and reduce the number of photolithography processes, anetching step may be performed with the use of a multi-tone mask which isa light-exposure mask through which light is transmitted to have aplurality of intensities. Since a resist mask formed using a multi-tonemask has a plurality of thicknesses and can be further changed in shapeby performing ashing, the resist mask can be used in a plurality ofetching steps to provide different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby one multi-tone mask. Thus, the number of light-exposure masks can bereduced and the number of corresponding photolithography processes canbe also reduced, so that the manufacturing process can be simplified.

Next, plasma treatment using a gas such as nitrous oxide (N₂O), nitrogen(N₂), or argon (Ar) is performed. By this plasma treatment, absorbedwater and the like attached to an exposed surface of the oxidesemiconductor layer are removed. Plasma treatment may be performed usinga mixture gas of oxygen and argon.

After the plasma treatment, an oxide insulating layer 416 that is incontact with part of the oxide semiconductor layer and serves as aprotective insulating film is formed without exposure to the air.

The oxide insulating layer 416 has a thickness of at least 1 nm and canbe formed as appropriate by a sputtering method or the like with whichimpurities such as water and hydrogen are not mixed into the oxideinsulating layer 416. When hydrogen is contained in the oxide insulatinglayer 416, entry of the hydrogen to the oxide semiconductor layer iscaused, thereby making a backchannel of the oxide semiconductor layer431 have a lower resistance (have n-type conductivity) and formingparasitic channels. Therefore, it is important that a formation methodin which hydrogen is not used is employed in order to form the oxideinsulating layer 416 containing as little hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is deposited asthe oxide insulating layer 416 by a sputtering method. The substratetemperature in film formation is higher than or equal to roomtemperature and lower than or equal to 300° C. and is 100° C. in thisembodiment. The silicon oxide film can be formed by a sputtering methodin a rare gas (typically argon) atmosphere, an oxygen atmosphere, or anatmosphere of a rare gas (typically argon) and oxygen. As a target, asilicon oxide target or a silicon target can be used. For example, thesilicon oxide film can be formed using a silicon target by a sputteringmethod in an atmosphere including oxygen and nitrogen.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample, at 250° C. to 350° C.) is performed in an inert gas atmosphere,a dry air atmosphere, or an oxygen gas atmosphere. For example, thesecond heat treatment is performed at 250° C. for one hour in a nitrogenatmosphere. Through the second heat treatment, part of the oxidesemiconductor layer (a channel formation region) is heated while beingin contact with the oxide insulating layer 416. Thus, oxygen is suppliedto part of the oxide semiconductor layer (the channel formation region).Moreover, in the vicinity of the interface between the source electrodelayer 415 a or the drain electrode layer 415 b and the oxidesemiconductor layer, oxygen is diffused into the source electrode layer415 a or the drain electrode layer 415 b. Accordingly, a source region414 a and a drain region 414 b are formed.

Through the above steps, the oxide semiconductor layer is subjected tothe heat treatment for dehydration or dehydrogenation, and then, part ofthe oxide semiconductor layer (the channel formation region) isselectively made to be in an oxygen excess state. Note that the part ofthe oxide semiconductor layer in an oxygen excess state is shown as aregion 413 in FIG. 12D. Through the above-described process, thetransistor 410 is formed.

Further, heat treatment may be performed at 100° C. to 200° C. for 1hour to 30 hours in the air. In this embodiment, the heat treatment isperformed at 150° C. for 10 hours. This heat treatment may be performedat a fixed heating temperature. Alternatively, the following change inthe heating temperature may be conducted plural times repeatedly: theheating temperature is increased from a room temperature to atemperature of 100° C. to 200° C. and then decreased to a roomtemperature.

A protective insulating layer may be further formed over the oxideinsulating layer 416. For example, a silicon nitride film is formed byan RF sputtering method. Since an RF sputtering method has highproductivity, it is preferably used as a method for forming theprotective insulating layer. As the protective insulating layer, aninorganic insulating film that does not include impurities such asmoisture, a hydrogen ion, and OH⁻ and blocks entry of these from theoutside is used. For example, a silicon nitride film, an aluminumnitride film, a silicon nitride oxide film, or an aluminum oxynitridefilm is used. In this embodiment, a protective insulating layer 403 isformed using a silicon nitride film as the protective insulating layer(see FIG. 12D).

The oxide semiconductor layer in the transistor 410 in this embodimentis an intrinsic (i-type) or substantially intrinsic oxide semiconductorlayer obtained by removal of hydrogen, which is an n-type impurity, forpurification so that the oxide semiconductor layer contains impuritiesother than the main component as little as possible. In other words, theoxide semiconductor layer in this embodiment is a purified i-type(intrinsic) semiconductor or a substantially intrinsic semiconductorobtained by removing impurities such as hydrogen and water as much aspossible, not by adding an impurity. This enables the Fermi level (Ef)to be at the same level as the intrinsic Fermi level (Ei).

It is said that an oxide semiconductor has a band gap (Eg) of 3.15 eVand an electron affinity (χ) of 4.3 eV. The work function of titanium(Ti) contained in the source electrode layer and the drain electrodelayer is substantially equal to the electron affinity (χ) of an oxidesemiconductor. In that case, a Schottky barrier for electrons is notformed at an interface between the metal and the oxide semiconductor.

For example, even when a transistor has a channel width W of 1×10⁴ μmand a channel length L of 3 μm, the off-state current can be 10⁻¹³ A orless and the subthreshold swing value (S value) can be 0.1 V/decade atroom temperature (with a 100-nm-thick gate insulating layer).

As described above, the oxide semiconductor is purified so as to containimpurities which are not its main components as little as possible;thus, the transistor 410 can operate in a favorable manner.

The above-described oxide semiconductor is a purified and electricallyintrinsic (i-type) oxide semiconductor made by the following manner: inorder to suppress variations in electrical characteristics, impuritiesthat cause variations, such as hydrogen, moisture, a hydroxyl group, andhydride (also referred to as hydrogen compound), are removedintentionally; and oxygen which is a main component of the oxidesemiconductor and is reduced in the step of removing impurities issupplied.

Therefore, it is preferable that the oxide semiconductor contain aslittle hydrogen as possible. Further, the purified oxide semiconductorhas very few carriers (close to zero) and the carrier density is lessthan 1×10¹²/cm³, preferably less than 1×10¹¹/cm³. In other words, thecarrier concentration of the oxide semiconductor layer is made as closeto zero as possible. Since the oxide semiconductor layer includesextremely few carriers, the off-state current of the transistor can bereduced. It is preferable that off-state current be as low as possible.The off-state current per channel width (W) of 1 μm of the transistor is100 aA/μm or less, preferably 10 zA/μm or less (zA: zeptoampere),further preferably 1 zA/μm or less. Further, since there is no pnjunction and no hot carrier degradation, electric characteristics of thetransistor are not adversely affected by them.

As described above, the off-state current can be extremely low in atransistor in which an oxide semiconductor that is purified bydrastically removing hydrogen contained therein is used for a channelformation region. In other words, in circuit design, the oxidesemiconductor layer can be regarded as an insulator when the transistoris off. On the other hand, when the transistor is on, the current supplycapability of the oxide semiconductor layer is expected to be higherthan that of a semiconductor layer formed of amorphous silicon.

Design is performed assuming that the off-state current of a thin filmtransistor formed using low-temperature polysilicon is approximately10000 times as high as that of a transistor formed using an oxidesemiconductor. Thus, a period for holding voltage of the transistorformed using an oxide semiconductor can be approximately 10000 times aslong as that of the thin film transistor formed using low-temperaturepolysilicon, when these transistors have an equivalent storagecapacitance (of about 0.1 pF). For example, when moving images aredisplayed at 60 frames per second, a holding period for one signalwriting can be approximately 160 seconds, which is 10000 times as longas that of the thin film transistor formed using low-temperaturepolysilicon. In this manner, a still image can be displayed on a displayportion even by less frequent writing of image signals.

Embodiment 6

In this embodiment, an example of a pixel and a method for driving thepixel will be described. Specifically, an example of a pixel thatincludes a display element with memory properties and an example of amethod for driving the pixel will be described.

FIG. 13A illustrates an example of a circuit diagram of a pixel. A pixel5450 includes a transistor 5451, a capacitor 5452, and a display element5453. A first terminal of the transistor 5451 is connected to a wiring5461. A second terminal of the transistor 5451 is connected to oneelectrode of the capacitor 5452 and one electrode of the display element5453 (also referred to as a pixel electrode). A gate of the transistor5451 is connected to a wiring 5462. The other electrode of the capacitor5452 is connected to a wiring 5463. The other electrode of the displayelement 5453 is connected to an electrode 5454 (e.g., a commonelectrode, a counter electrode, or a cathode electrode).

Note that an electrode 5455 refers to one electrode of the displayelement 5453.

When FIG. 13A is compared to FIG. 10A, the transistor 5451 correspondsto the transistor 801; the display element 5453, the display element802; the capacitor 5452, the capacitor 804; the wiring 5462, the wiring104 a; and the wiring 5461, the wiring 103 a.

The display element 5453 preferably has memory properties. Examples ofthe display element 5453 and a method for driving the display element5453 are microcapsule electrophoresis, microcup electrophoresis,horizontal electrophoresis, vertical electrophoresis, twisting ball,liquid powder display, electronic liquid powder (registered trademark),a cholesteric liquid crystal element, a chiral nematic liquid crystal,an anti-ferroelectric liquid crystal, and a polymer dispersed liquidcrystal.

Since a voltage applied to the display element 5453 is very high, atransistor for driving the display element 5453 needs to have highwithstand voltage. Moreover, the off-state current of the transistorneeds to be low even when such a high voltage is applied. By using atransistor including an oxide semiconductor as the transistor, thetransistor can have high withstand voltage and low off-state current.

FIG. 13B is a cross-sectional view of a pixel using microcapsuleelectrophoresis. A plurality of microcapsules 5480 are placed between anelectrode 5454 and an electrode 5455. The plurality of microcapsules5480 are fixed by a resin 5481. The resin 5481 functions as a binder.The resin 5481 preferably has light-transmitting properties. A spaceformed by the electrode 5454, the electrode 5455, and the microcapsule5480 can be filled with a gas such as air or an inert gas. In such acase, a layer including a glue, an adhesive, or the like is preferablyformed on one or both of the electrodes 5454 and 5455 to fix themicrocapsules 5480.

The microcapsule 5480 includes a film 5482, a liquid 5483, particles5484, and particles 5485. The liquid 5483, the particles 5484, and theparticles 5485 are sealed in the film 5482. The film 5482 haslight-transmitting properties. The liquid 5483 functions as a dispersionliquid. The particles 5484 and the particles 5485 can be dispersed inthe film 5482 by the liquid 5483. It is preferable that the liquid 5483have light-transmitting properties and be not colored. The particle 5484and the particle 5485 have different colors. For example, it ispreferable that one of the particle 5484 and the particle 5485 be blackand the other of the particle 5484 and the particle 5485 be white. Notethat the particle 5484 and the particle 5485 are charged so that theirelectric charge densities or polarities are different from each other.For example, one of the particle 5484 and the particle 5485 ispositively charged and the other of the particle 5484 and the particle5485 is negatively charged. Thus, when a potential difference occursbetween the electrode 5454 and the electrode 5455, the particle 5484 andthe particle 5485 move in accordance with the direction of electricfields. Accordingly, the reflectance of the display element 5453 ischanged, so that the gray level can be controlled. Note that thestructure of the microcapsule 5480 is not limited to the above-describedstructure. For example, the liquid 5483 can be colored. As anotherexample, as particles sealed in the film 5482, one kind of particles orthree kinds or more of particles can be used. As another example, colorsof the particle 5484 and the particle 5485 can be selected from red,green, blue, cyan, magenta, yellow emerald green, vermillion, and thelike in addition to white and black.

The film 5482 is formed using a light-transmitting material (e.g., apolymer resin such as an acrylic resin (e.g., poly(methyl methacrylate)and poly(ethyl methacrylate)), a urea resin, or gum arabic), forexample. Note that the film 5482 is preferably gelatinous. By using sucha film 5482, the plasticity, bending strength, mechanical strength, andthe like can be improved, leading to improvement in flexibility.Alternatively, the microcapsules 5480 can be uniformly arranged with nogap therebetween over a substrate such as film.

A light-transmitting oily liquid is preferably used as the liquid 5483.Specific examples of the liquid 5483 are an alcohol-based solvent (e.g.,methanol and ethanol), ester (e.g., ethyl acetate and butyl acetate),aliphatic hydrocarbon (e.g., ketone such as acetone, methyl ethylketone, and methyl isobutyl ketone; pentane, hexane, and octane),alicyclic hydrocarbon (e.g., cyclohexane and methylcyclohexane),aromatic hydrocarbon such as benzene having a long-chain alkyl group(e.g., benzene, toluene, and xylene), halogenated hydrocarbon (e.g.,methylene chloride and chloroform), calboxylate salt, water, and otherkinds of oils. Other examples of the liquid 5483 are a mixture of two ormore of the above-described materials, a combination of a surface activeagent or the like and one of the above materials, and a combination of asurface active agent or the like and a mixture of two or more of theabove materials.

Each of the particle 5484 and the particle 5485 is formed using apigment. The pigments included in the particle 5484 and the particle5485 preferably have different colors. For example, it is preferablethat the particle 5484 be formed using a black pigment and the particle5485 be formed using a white pigment. Examples of the black pigment areaniline black and carbon black. Examples of the white pigment aretitanium dioxide, zinc white (zinc oxide), and antimony trioxide. Notethat it is possible to add, to the above-described pigment, a chargecontrolling agent (e.g., electrolyte, a surface active agent, metallicsoap, resin, rubber, oil, varnish, or a compound), a dispersing agent(e.g., a titanium-based coupling agent, an aluminum-based couplingagent, or a silane-based coupling agent), a lubricant agent, astabilizing agent, or the like.

FIG. 14A is a cross-sectional view of a pixel in the case where atwisting ball display method is used for the display element 5453. Inthe twisting ball display method, the reflectance is changed by rotationof a display element in order to control the gray level. The differencefrom FIG. 13B is that instead of the microcapsule 5480, a twisting ball5486 is placed between the electrode 5454 and the electrode 5455. Thetwisting ball 5486 includes a particle 5487 and a cavity 5488 formedaround the particle 5487. The particle 5487 is a spherical particle inwhich a surface of one hemisphere is colored in a given color and asurface of the other hemisphere is colored in a different color. Here,the particle 5487 has a white hemisphere and a black hemisphere. Notethat there is a difference in electric charge density or polaritybetween the two hemispheres. For that reason, by generating a potentialdifference between the electrode 5454 and the electrode 5455, theparticle 5487 can be rotated in accordance with the direction ofelectric fields. The cavity 5488 is filled with a liquid. As the liquid,a liquid similar to the liquid 5483 can be used. Note that the structureof the twisting ball 5486 is not limited to the structure illustrated inFIG. 14A. For example, the twisting ball 5486 can be a cylinder, anellipse, or the like.

FIG. 14B is a cross-sectional view of a pixel in the case where amicrocup electrophoresis method is used for the display element 5453. Amicrocup array can be formed in the following manner: a microcup 5491that is formed using a UV curable resin or the like and has a pluralityof recessed portions is filled with charged pigment particles 5493dispersed in a dielectric solvent 5492, and sealing is performed with asealing layer 5494. An adhesive layer 5495 is preferably formed betweenthe sealing layer 5494 and the electrode 5455. As the dielectric solvent5492, a colorless solvent can be used or a colored solvent of red, blue,or the like can be used. This embodiment shows the case where one kindof charged pigment particles is used; alternatively, two or more kindsof charged pigment particles may be used. The microcup has a wall bywhich cells are separated, and thus has sufficiently high resistance toshock and pressure. Moreover, since the components of the microcup aretightly sealed, adverse effects due to change in environment can bereduced.

FIG. 14C is a cross-sectional view of a pixel in the case where anelectronic liquid powder (registered trademark) display method is usedfor the display element 5453. The electronic liquid powder has fluidityand is a substance having properties of fluid and properties of aparticle. In this method, cells are separated by partitions 5504, andelectronic liquid powders 5502 and electronic liquid powders 5503 areplaced in the cell. As the electronic liquid powder 5502 and theelectronic liquid powder 5503, a white particle and a black particle arepreferably used. Note that the kinds of the electronic liquid powders5502 and 5503 are not limited thereto. For example, colored particles oftwo colors which are not white and black can be used as the electronicliquid powders 5502 and 5503. As another example, one of the electronicliquid powder 5502 and the electronic liquid powder 5503 can be omitted.

As illustrated in FIG. 13A, a signal is input to the wiring 5461.Specifically, a signal for controlling the gray level of the displayelement 5453 (e.g., a video signal) is input to the wiring 5461.Accordingly, the wiring 5461 has a function of a signal line or a sourcesignal line (also referred to as a video signal line or a source line).A signal is input to the wiring 5462. Specifically, a signal forcontrolling a conduction state of the transistor 5451 (e.g., a gatesignal, a scan signal, or a selection signal) is input to the wiring5462. Accordingly, the wiring 5462 has a function of a signal line or agate signal line (also referred to as a scan signal line or a gateline). A predetermined voltage is supplied to the wiring 5463. Thewiring 5463 is connected to the capacitor 5452. Accordingly, the wiring5463 has a function of a power supply line or a capacitor line. Apredetermined voltage is supplied to the electrode 5454. The electrode5454 is often shared with a plurality of pixels or all the pixels.Accordingly, the electrode 5454 has a function of a common electrode(such as a counter electrode or a cathode electrode).

Note that the signals or voltages input to the wirings 5461 to 5463 andthe electrode 5454 are not limited to the above, and various othersignals or voltages can be input. For example, a signal can be input tothe wiring 5463. Thus, the potential of the electrode 5455 can becontrolled, so that the amplitude voltage of a signal input to thewiring 5461 can be reduced. Accordingly, the wiring 5463 can have afunction of a signal line. As another example, by changing a voltagesupplied to the electrode 5454, a voltage applied to the display element5453 can be adjusted. Thus, the amplitude voltage of a signal input tothe wiring 5461 can be reduced.

The transistor 5451 has a function of controlling electrical continuitybetween the wiring 5461 and the electrode 5455, a function ofcontrolling the timing of supplying the potential of the wiring 5461 tothe electrode 5455, and/or a function of controlling the timing ofselecting the pixel 5450. In such a manner, the transistor 5451 has afunction of a switch or a selection transistor. The transistor 5451 isan n-channel transistor. For that reason, the transistor 5451 is turnedon when an H-level signal is input to the wiring 5462, and is turned offwhen an L-level signal is input to the wiring 5462. Note that transistor5451 is not limited to an n-channel transistor and can be a p-channeltransistor. In that case, the transistor 5451 is turned on when anL-level signal is input to the wiring 5462, and is turned off when anH-level signal is input to the wiring 5462. The capacitor 5452 has afunction of holding the potential difference between the electrode 5455and the wiring 5463, and/or a function of keeping the potential of theelectrode 5455 at a predetermined value. Thus, a voltage can continue tobe applied to the display element 5453 even when the transistor 5451 isoff. In such a manner, the capacitor 5452 has a function of a storagecapacitor. Note that functions of the transistor 5451 and the capacitor5452 are not limited to the above, and the transistor 5451 and thecapacitor 5452 can have various other functions.

Next, the operation of the pixel in this embodiment will be roughlydescribed. The gray level of the display element 5453 is controlled byapplying a voltage to the display element 5453 so that an electric fieldis generated in the display element 5453. A voltage applied to thedisplay element 5453 is controlled by controlling the potential of theelectrode 5454 and the potential of the electrode 5455. Specifically,the potential of the electrode 5454 is controlled by controlling avoltage applied to the electrode 5454. The potential of the electrode5455 is controlled by controlling a signal input to the wiring 5461. Thesignal input to the wiring 5461 is supplied to the electrode 5455 whenthe transistor 5451 is turned on.

Note that the gray level of the display element 5453 can be controlledby controlling at least one of the intensity of electric fields appliedto the display element 5453, the direction of electric fields applied tothe display element 5453, the time during which electric fields areapplied to the display element 5453, and the like. Note that the graylevel of the display element 5453 can be maintained by not generating apotential difference between the electrode 5454 and the electrode 5455.

Next, an example of the operation of the pixel in this embodiment willbe described. The timing chart in FIG. 15A shows a period T including aselection period and a non-selection period. The period T is a periodfrom the start of a selection period until the start of the nextselection period.

In the selection period, an H-level signal is input to the wiring 5462,so that the potential of the wiring 5462 (shown as a potential V5462) isat H level. For that reason, the transistor 5451 is turned on, so thatelectrical continuity is established between the wiring 5461 and theelectrode 5455. Thus, a signal input to the wiring 5461 is supplied tothe electrode 5455 through the transistor 5451, and the potential of theelectrode 5455 (shown as a potential V5455) becomes a value equal to thesignal input to the wiring 5461. At this time, the capacitor 5452 holdsa potential difference between the electrode 5455 and the wiring 5463.In the non-selection period, an L-level signal is input to the wiring5462, so that the potential of the wiring 5462 is at L level. For thatreason, the transistor 5451 is turned off, and electrical continuitybetween the wiring 5461 and the electrode 5455 is broken. Then, theelectrode 5455 is set in a floating state. At this time, the capacitor5452 holds the potential difference between the electrode 5455 and thewiring 5463 in the selection period. For that reason, the potential ofthe electrode 5455 remains equal to the signal input to the wiring 5461in the selection period. In such a manner, in the non-selection period,a voltage can continue to be applied to the display element 5453 evenwhen the transistor 5451 is off. As described above, by controlling asignal input to the wiring 5461 in the selection period, a voltageapplied to the display element 5453 can be controlled. That is, the graylevel of the display element 5453 can be controlled by controlling asignal input to the wiring 5461 in the selection period.

The potential of the electrode 5455 in the non-selection period may bedifferent from the signal input to the wiring 5461 in the selectionperiod because of adverse effects of at least one of the off-statecurrent of the transistor 5451, feedthrough of the transistor 5451,charge injection of the transistor 5451, and the like.

As illustrated in FIG. 15B, the potential of the electrode 5455 can beequal to that of the electrode 5454 in part of the selection period.Accordingly, even if the same signal continues to be input to the pixel5450 every time the pixel 5450 is selected, the intensity of electricfields applied to the display element 5453 can be changed by changingthe potential of the electrode 5455 in part of the selection period.Therefore, afterimages can be reduced; the response speed can beincreased; or variations in response speed between pixels can be reducedso that unevenness or afterimages can be prevented. In order to realizesuch a driving method, the selection period is preferably divided into aperiod T1 and a period T2. In the period T1, the potential of the signalinput to the wiring 5461 is preferably equal to that of the electrode5454. In the period T2, the signal input to the wiring 5461 preferablyhas various values in order to control the gray level of the displayelement 5453. Note that when the period T1 is too long, the time duringwhich a signal for controlling the gray level of the display element5453 is written into the pixel 5450 becomes short. Therefore, the periodT1 is preferably shorter than the period T2. Specifically, the period T1accounts for preferably 1 to 20%, more preferably 3 to 15%, furtherpreferably 5 to 10% of the selection period.

Next described is an example of the operation of the pixel in thisembodiment, in which the gray level of the display element 5453 iscontrolled by the time during which a voltage is applied to the displayelement 5453. The timing chart in FIG. 15C shows a period Ta and aperiod Tb. The period Ta includes N periods T (N is a natural number).The N periods T are similar to the period T illustrated in FIG. 15A orFIG. 15B. The period Ta is a period for changing the gray level of thedisplay element 5453 (e.g., an address period, a writing period, or animage rewriting period). The period Tb is a period during which the graylevel of the display element 5453 in the period Ta is held (i.e., aholding period).

A voltage V0 is supplied to the electrode 5454, so that the electrode5454 is at a potential V0. A signal having at least three values isinput to the wiring 5461. Three potentials of the signal are a potentialVH (VH>V0), the potential V0, and a potential VL (VL<V0). Accordingly,the potential VH, the potential V0, and the potential VL are applied tothe electrode 5455.

In each of the N periods T in the period Ta, by controlling a potentialapplied to the electrode 5455, a voltage applied to the display element5453 can be controlled. For example, when the potential VH is applied tothe electrode 5455, the potential difference between the electrode 5454and the electrode 5455 becomes VH−VL. Thus, a positive voltage can beapplied to the display element 5453. When the potential V0 is applied tothe electrode 5455, the potential difference between the electrode 5454and the electrode 5455 becomes zero. Thus, zero voltage can be appliedto the display element 5453. When the potential VL is applied to theelectrode 5455, the potential difference between the electrode 5454 andthe electrode 5455 becomes VL−VH. Thus, a negative voltage can beapplied to the display element 5453. As described above, in the periodTa, the positive voltage (VH−VL), the negative voltage (VL−VH), and zerovoltage can be applied to the display element 5453 in a variety oforders. Thus, the gray level of the display element 5453 can be minutelycontrolled; afterimages can be reduced; or the response speed can beincreased.

Note that in this embodiment, when a positive voltage is applied to thedisplay element 5453, the gray level of the display element 5453 isclose to black (also referred to as a first gray level). When a negativevoltage is applied to the display element 5453, the gray level of thedisplay element 5453 is close to white (also referred to as a secondgray level). When zero voltage is applied to the display element 5453,the gray level of the display element 5453 is maintained.

In the period Tb, a signal input to the wiring 5461 is not written intothe pixel 5450. Therefore, a potential applied to the electrode 5455 inthe Nth period T in the period Ta continues to be applied in the periodTb. Specifically, in the period Tb, the gray level of the displayelement 5453 is preferably maintained by not generating electric fieldsin the display element 5453. For that reason, in the Nth period T in theperiod Ta, the potential V0 is preferably applied to the electrode 5455.Thus, the potential V0 is applied to the electrode 5455 also in theperiod Tb, so that zero voltage is applied to the display element 5453.In such a manner, the gray level of the display element 5453 can bemaintained.

Note that as the gray level to be subsequently expressed by the displayelement 5453 is closer to the first gray level, the time during whichthe potential VH is applied to the electrode 5455 is preferably longerin the period Ta. Alternatively, the frequency of application of thepotential VH to the electrode 5455 is preferably higher in the N periodsT. Alternatively, in the period Ta, it is preferable to increase a timeobtained by subtracting the time during which the potential VL isapplied to the electrode 5455 from the time during which the potentialVH is applied to the electrode 5455. Further alternatively, in the Nperiods T, it is preferable to increase a frequency obtained bysubtracting the frequency of application of the potential VL to theelectrode 5455 from the frequency of application of the potential VH tothe electrode 5455.

In addition, as the gray level to be subsequently expressed by thedisplay element 5453 is closer to the second gray level, the time duringwhich the potential VL is applied to the electrode 5455 is preferablylonger in the period Ta. Alternatively, the frequency of application ofthe potential VL to the electrode 5455 is preferably higher in the Nperiods T. Alternatively, in the period Ta, it is preferable to increasea time obtained by subtracting the time during which the potential VH isapplied to the electrode 5455 from the time during which the potentialVL is applied to the electrode 5455. Further alternatively, in the Nperiods T, it is preferable to increase a frequency obtained bysubtracting the frequency of application of the potential VH to theelectrode 5455 from the frequency of application of the potential VL tothe electrode 5455.

In the period Ta, a combination of potentials (the potential VH, thepotential V0, and the potential VL) applied to the electrode 5455 candepend not only on the gray level to be subsequently expressed by thedisplay element 5453, but also on the gray level that has been expressedby the display element 5453. For that reason, if a different gray levelhas been expressed by the display element 5453, a combination ofpotentials applied to the electrode 5455 may vary even when the graylevel to be subsequently expressed by the display element 5453 is thesame.

For example, in the period Ta for expressing the gray level that hasbeen expressed by the display element 5453, the time during which thepotential VL is applied to the electrode 5455 is preferably longer inthe period Ta in any of the following cases: the case where the timeduring which the potential VH is applied to the electrode 5455 islonger; the case where a time obtained by subtracting the time duringwhich the potential VL is applied to the electrode 5455 from the timeduring which the potential VH is applied to the electrode 5455 islonger; the case where the frequency of application of the potential VHto the electrode 5455 is higher in the N periods T; or the case where afrequency obtained by subtracting the frequency of application of thepotential VL to the electrode 5455 from the frequency of application ofthe potential VH to the electrode 5455 is higher in the N periods T.Alternatively, the frequency of application of the potential VL to theelectrode 5455 is preferably higher in the N periods T. Alternatively,in the period Ta, it is preferable to increase a time obtained bysubtracting the time during which the potential VH is applied to theelectrode 5455 from the time during which the potential VL is applied tothe electrode 5455. Further alternatively, in the N periods T, it ispreferable to increase a frequency obtained by subtracting the frequencyof application of the potential VH to the electrode 5455 from thefrequency of application of the potential VL to the electrode 5455. Insuch a manner, afterimages can be reduced.

As another example, in the period Ta for expressing the gray level thathas been expressed by the display element 5453, the time during whichthe potential VH is applied to the electrode 5455 is preferably longerin the period Ta in any of the following cases: the case where the timeduring which the potential VL is applied to the electrode 5455 islonger; the case where a time obtained by subtracting the time duringwhich the potential VH is applied to the electrode 5455 from the timeduring which the potential VL is applied to the electrode 5455 islonger; the case where the frequency of application of the potential VLto the electrode 5455 is higher in the N periods T; or the case where afrequency obtained by subtracting the frequency of application of thepotential VH to the electrode 5455 from the frequency of application ofthe potential VL to the electrode 5455 is higher in the N periods T.Alternatively, the frequency of application of the potential VH to theelectrode 5455 is preferably higher in the N periods T. Alternatively,in the period Ta, it is preferable to increase a time obtained bysubtracting the time during which the potential VL is applied to theelectrode 5455 from the time during which the potential VH is applied tothe electrode 5455. Further alternatively, in the N periods T, it ispreferable to increase a frequency obtained by subtracting the frequencyof application of the potential VL to the electrode 5455 from thefrequency of application of the potential VH to the electrode 5455. Insuch a manner, afterimages can be reduced.

The N periods T have the same length; however, the length of the Nperiods T is not limited thereto and the lengths of at least two of theN periods T can be different from each other. It is particularlypreferable that the length of the N periods T be weighted. For example,in the case where N is 4 and the length of the first period T is denotedby a time h, the length of the second period T is preferably a time h×2,the length of the third period T is preferably a time h×4, and thelength of the fourth period T is preferably a time h×8. When the lengthof the N periods T is weighted in such a manner, the frequency ofselection of the pixels 5450 can be reduced and the time during which avoltage is applied to the display element 5453 can be minutelycontrolled. Thus, power consumption can be reduced.

The potential VH and the potential VL can be selectively applied to theelectrode 5454. In this case, it is preferable that the potential VH andthe potential VL be selectively applied also to the electrode 5455. Forexample, in the case where the potential VH is applied to the electrode5454, zero voltage is applied to the display element 5453 when thepotential VH is applied to the electrode 5455, whereas a negativevoltage is applied to the display element 5453 when the potential VL isapplied to the electrode 5455. On the other hand, in the case where thepotential VL is applied to the electrode 5454, a positive voltage isapplied to the display element 5453 when the potential VH is applied tothe electrode 5455, whereas zero voltage is applied to the displayelement 5453 when the potential VL is applied to the electrode 5455. Insuch a manner, the signal input to the wiring 5461 can have two values(i.e., the signal can be a digital signal). For that reason, it ispossible to simplify a circuit that outputs a signal to the wiring 5461.

In the period Tb or part of the period Tb, it is possible not to input asignal to the wiring 5461 and the wiring 5462. That is, the wiring 5461and the wiring 5462 can be set in a floating state. Moreover, in theperiod Tb or part of the period Tb, it is possible not to input a signalto the wiring 5463. That is, the wiring 5463 can be set in a floatingstate. Furthermore, in the period Tb or part of the period Tb, it ispossible not to supply a voltage to the electrode 5454. That is, theelectrode 5454 can be set in a floating state.

Embodiment 7

In this embodiment, examples of electronic devices will be described.

FIGS. 16A to 16H and FIGS. 17A to 17D each illustrate an electronicdevice. These electronic devices can include a housing 5000, a displayportion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005(including a power switch or an operation switch), a connection terminal5006, a sensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 5008, and the like.

FIG. 16A illustrates a mobile computer that can include a switch 5009,an infrared port 5010, and the like in addition to the above-describedcomponents. FIG. 16B illustrates a portable image reproducing device(e.g., a DVD reproducing device) provided with a memory medium, and theimage reproducing device can include a second display portion 5002, amemory medium reading portion 5011, and the like in addition to theabove components. FIG. 16C illustrates a goggle-type display that caninclude second display portion 5002, a support portion 5012, an earphone5013, and the like in addition to the above components. FIG. 16Dillustrates a portable game machine that can include the memory mediumreading portion 5011 and the like in addition to the above components.FIG. 16E illustrates a projector that can include a light source 5033, aprojector lens 5034, and the like in addition to the above components.FIG. 16F illustrates a portable game machine that can include the seconddisplay portion 5002, the memory medium reading portion 5011, and thelike in addition to the above components. FIG. 16G illustrates atelevision set that can include a tuner, an image processing portion,and the like in addition to the above components. FIG. 16H illustrates aportable television receiver that can include a charger 5017 capable oftransmitting and receiving signals and the like in addition to the abovecomponents. FIG. 17A illustrates a display that can include a supportbase 5018 and the like in addition to the above-described components.FIG. 17B illustrates a camera that can include an external connectionport 5019, a shutter button 5015, an image receiving portion 5016, andthe like in addition to the above components. FIG. 17C illustrates acomputer that can include a pointing device 5020, the externalconnection port 5019, a reader/writer 5021, and the like in addition tothe above components. FIG. 17D illustrates a mobile phone that caninclude an antenna 5014, a tuner of one-segment (1 seg digital TVbroadcasts) partial reception service for mobile phones and mobileterminals, and the like in addition to the above components.

The electronic devices illustrated in FIGS. 16A to 16H and FIGS. 17A to17D can have a variety of functions.

The electronic device described in this embodiment includes a displayportion for displaying some kind of information.

Next, applications of a semiconductor device will be described.

FIG. 17E illustrates an example in which a semiconductor device isincorporated in a building structure. FIG. 17E illustrates a housing5022, a display portion 5023, a remote controller 5024 which is anoperation portion, a speaker 5025, and the like. The semiconductordevice is incorporated in the building as a wall-hanging type and can beprovided without requiring a large space.

FIG. 17F illustrates another example in which a semiconductor device isincorporated in a building. A display panel 5026 is integrated with aprefabricated bath 5027, so that a person who takes a bath can watch thedisplay panel 5026.

Note that although the wall and the prefabricated bath are given asexamples of the building, this embodiment is not limited to theseexamples and the semiconductor device can be provided in a variety ofbuildings.

Next, examples in which a semiconductor device is incorporated with amoving object will be described.

FIG. 17G illustrates an example in which a semiconductor device isprovided in a car. A display panel 5028 is provided in a body 5029 ofthe car and can display information related to the operation of the caror information input from inside or outside of the car on demand. Notethat a navigation function may be provided.

FIG. 17H illustrates an example in which a semiconductor device isincorporated in a passenger airplane. FIG. 17H shows a usage patternwhen a display panel 5031 is provided for a ceiling 5030 above a seat ofthe airplane. The display panel 5031 is integrated with the ceiling 5030through a hinge portion 5032, and a passenger can watch the displaypanel 5031 by extending and contracting the hinge portion 5032. Thedisplay panel 5031 has a function of displaying information whenoperated by the passenger.

Note that although the body of the car and the body of the plane aregiven as examples of the moving body, this embodiment is not limited tothese examples. The display device can be provided for a variety ofmoving bodies such as a two-wheel motor vehicle, a four-wheel vehicle(including a car, bus, and the like), a train (including a monorail, arailway, and the like), and a ship.

In such an electronic device, the formation of a circuit with the use ofa transistor that includes an oxide semiconductor and has a lowoff-state current can prevent unnecessary current from being leaked intothe circuit. For that reason, the circuit is likely to operate normally.As a result, accurate display can be performed.

This application is based on Japanese Patent Application serial no.2010-028285 filed with Japan Patent Office on Feb. 11, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A liquid crystal display device comprising: a pixel portion including a first pixel and a second pixel each performing red display; and a circuit configured to inspect the pixel portion, the circuit including a third transistor and a fourth transistor, wherein the first pixel includes a first transistor and a first liquid crystal element, wherein the second pixel includes a second transistor and a second liquid crystal element, wherein one of source and drain of the first transistor is electrically connected to a first signal line and the other of source and drain of the first transistor is electrically connected to the first liquid crystal element, wherein one of source and drain of the second transistor is electrically connected to a second signal line and the other of source and drain of the second transistor is electrically connected to the second liquid crystal element, wherein gates of the third transistor and the fourth transistor are electrically connected to each other, wherein one of source and drain of the third transistor is electrically connected to the first signal line, wherein one of source and drain of the fourth transistor is electrically connected to the second signal line, wherein each of the first transistor, the second transistor, the third transistor, and the fourth transistor includes an oxide semiconductor layer, an oxide insulating layer over and in contact with the oxide semiconductor layer, and a silicon nitride film over and in contact with the oxide insulating layer.
 2. The liquid crystal display device according to claim 1, wherein the other of source and drain of the third transistor and the other of source and drain of the fourth transistor are electrically connected to different wirings.
 3. The liquid crystal display device according to claim 1, wherein an off-state current per channel width of 1 μm of each of the first transistor, the second transistor, the third transistor, and the fourth transistor is 1 zA/μm.
 4. The liquid crystal display device according to claim 1, wherein carrier density of the oxide semiconductor layer is less than 1×10¹²/cm³.
 5. An electronic device including the liquid crystal display device according to claim 1 and an operation switch.
 6. A liquid crystal display device comprising: a pixel portion including a first pixel and a second pixel each performing red display; and a circuit configured to inspect the pixel portion, the circuit including a third transistor and a fourth transistor, wherein the first pixel includes a first transistor and a first liquid crystal element, wherein the second pixel includes a second transistor and a second liquid crystal element, wherein one of source and drain of the first transistor is electrically connected to a first signal line and the other of source and drain of the first transistor is electrically connected to the first liquid crystal element, wherein one of source and drain of the second transistor is electrically connected to a second signal line and the other of source and drain of the second transistor is electrically connected to the second liquid crystal element, wherein gates of the third transistor and the fourth transistor are electrically connected to each other, wherein one of source and drain of the third transistor is electrically connected to the first signal line, wherein one of source and drain of the fourth transistor is electrically connected to the second signal line, wherein each of the first transistor, the second transistor, the third transistor, and the fourth transistor includes an oxide semiconductor layer, a silicon oxide film over and in contact with the oxide semiconductor layer, and a silicon nitride film over and in contact with the silicon oxide film.
 7. The liquid crystal display device according to claim 6, wherein the other of source and drain of the third transistor and the other of source and drain of the fourth transistor are electrically connected to different wirings.
 8. The liquid crystal display device according to claim 6, wherein an off-state current per channel width of 1 μm of each of the first transistor, the second transistor, the third transistor, and the fourth transistor is 1 zA/μm.
 9. The liquid crystal display device according to claim 6, wherein carrier density of the oxide semiconductor layer is less than 1×10¹²/cm³.
 10. An electronic device including the liquid crystal display device according to claim 6 and an operation switch. 